1. Field of the Invention
The present invention relates to a cold cathode which can serve as an electron emission source and more particularly to a field emission type cold cathode which emits electrons from pointed apexes thereof and an electron tube equipped with the said cold cathode.
2. Description of the Prior Art
A structure of a cold cathode element, field emitter arrays (abbreviated FEAs hereinafter), was already described in Journal of Applied Physics, Vol.39, No.7, p.3504, 1968, wherein minute cold cathodes are arranged in arrays, each cold cathode comprising an emitter in the form of a miniature cone and a gate electrode which is formed very close to the emitter and has a current control function and a function to draw out currents from the emitter.
FEAs (cold cathode element) in such a structure are called Spindt-type cold cathodes after the developer thereof and have merits such as a capability to attain a high current density, compared with thermionic cathodes, and a narrow velocity distribution of emitted electrons therefrom.
Further, FEAs produce less current noise, in comparison with single field emitters utilized in conventional electron microscopes, and are characterized by operating with low voltages ranging from several tens to 200 V.
Further, while the single field emitters utilized in electron microscopes require a vacuum with an ultra-high degree of the order of 10.sup.-8 Pa, FEAs are characterized by a capability to operate even in a sealed glass tube in a vacuum environment of 10.sup.-4 .about.10.sup.-6 Pa, owing to the structure in which gate electrodes are placed very close to emitters and to having a plurality of emitters therein.
FIG. 5 shows a cross-sectional view of the main part structure of the conventional Spindt-type FEAs (cold cathode element). On a silicon substrate 101, a plurality of emitters 102 in the form of a miniature cone with a height of approximately 1 .mu.m are formed by the vacuum vapor deposition method and around each emitter 102, a gate electrode 103 and an insulating layer 104 are formed.
The substrate 101 and the emitter 102 are electrically connected and, as a sandwich voltage for the substrate 101, the emitter 102 and the gate electrode 103, a DC voltage of approximately 100 V is applied to the gate electrode 103, being positively polarized with respect to the substrate and the emitter. The distance between the substrate 101 and the gate electrode 103 is approximately 1 .mu.m and a diameter of the opening in the gate electrode is also as narrow as 1 .mu.m, and, moreover, the apex of the emitter 102 is formed to be sharply pointed so that a strong electric field is applied to the apex of the emitter 102.
When the intensity of this electric field becomes equal to or more than 2.about.5.times.10.sup.7 V/cm, electrons are sent forth from the apex of the emitter 102 and an electric current of 0.1.about.several tens of .mu.A per emitter is obtained. By arranging in arrays a plurality of minute cold cathodes with such a structure, a plane-shaped cathode from which a high electric current can flow out is constituted.
As for the application of such Spindt-type cold cathodes, the flat screen display device, the electron tubes such as the camera tube, the microwave tube and the Braun tube and the electron sources for various sensors have been proposed.
In general, FEAs (cold cathode element) have a structure wherein, by narrowing the gap between the emitter and the gate electrode up to .mu.m.about.sub .mu.m, and further, by making the apex of the emitters sharply pointed, a strong electric field is applied to the apex of the emitter. Consequently, when the degree of vacuum in operation is lowered, electric discharge is liable to occur between the emitter and the gate electrode.
A prolonged electric discharge melts the emitter and then leads to a breakdown through melting even the surrounding gate electrode and insulating layer, resulting in a short-circuit between the emitter and the gate electrode.
In order to prevent such breakdown by short-circuits between the emitters and gate electrodes due to a prolonged electric discharge, a method to form a resistive layer right under the emitters on the substrate and make the conductive pattern for supplying power to the emitters as a meshed form is disclosed in U.S. Pat. No. 4,940,916. However, this method requires to make the conductive pattern as a meshed form so that the density of elementary emitters cannot be increased.
Further, because an emitter located in the central region of this mesh has a higher resistance than an emitter located on the edge of the mesh, it becomes difficult to emit electrons, which is a clear disadvantage. In order to overcome these disadvantages described above, and at the same time suppressing prolonged electrical discharge, the present applicants have already disclosed (Japanese Patent Application No. 133959/1996) a field emission type cold cathode device which is characterized by having insulating layers surrounding areas right under each emitter, as shown in FIG. 6, wherein the said insulating layers are formed with an insulator by filling up trenches which are set in a semiconductor substrate and surround respective areas right under each emitter.
In such a device, because the area right under each emitter is surrounded by the said insulating layer, respectively, carriers cannot spread over the surface of the semiconductor substrate or lower the resistance, and, as a result, even if the electric discharge takes place, the value of the resistance of the semiconductor substrate can be kept almost constant and thereby, the peak current of electric discharge can be controlled.
Further, this resistance is divided into respective insulating layers surrounding an area right under each emitter so that a voltage drop taking place to this resistance in normal operation is very small (1/the number of the division), compared with the aforementioned resistive layer. Further, no need to keep horizontal distances like in the resistive layer allows increasing the density of elementary emitters.
However, in a field emission type cold cathode device, as shown in FIG. 6, since the substrate is divided into respective areas right under each emitter surrounded by an insulating layer (Block group), the voltage drop in the said surrounded area is certainly small, but, in the normal operation, when electrons are sent forth from an emitter, each being separated from the others by a respective insulating layer surrounding the area right under this emitter, a depletion region is formed along the wall of the insulating layer, as shown in FIG. 7, resulting in an increase in resistance of the said surrounded and divided area.
This depletion region is generated, via the wall of the insulating layer, by a difference of the electric potential from that of the adjacent substrate area where no emitter is formed. That is, when electrons are sent forth from an emitter, a resistance of the said surrounded and divided area in the outer-most area below a formed emitter which causes a voltage drop to take place so that the electric potential right under the emitter goes up, which causes a difference of electric potential from that of the adjacent substrate where no emitter is formed, via the wall of the insulating layer, and thereby a depletion region is formed. As the emission increases, this phenomenon becomes more apparent and the emission current may saturate, as shown in FIG. 8.
As a result, in the emitter formed area group, each of which is surrounded by respective insulating layers, non-uniformity of the emission currents arises between the inner divided area sharing a surrounded insulating layer with an insulating layer surrounding another emitter group and the outer-most divided area not sharing an insulating layer with an insulating layer surrounding another emitter group. Therefore, such cold cathodes have disadvantages that, when applied to the flat screen display device, they may cause non-uniformity of brightness of the screen within the display area, and make the image quality very poor.
Further, according to an application by the present applicants (Japanese Patent Application No. 80840/1997), in order to solve the problem of the non-uniformity of emission resulting from the afore-mentioned difference between depletion region, methods to increase the width of insulating layers surrounding the outer-most divided areas, or alternatively to enlarge the size of the outer-most divided areas are disclosed, but this method has another disadvantage that, during the etch-back step which is normally carried out after burying the trenches in manufacturing a field emission type cold cathode, etching tends to proceed from sections near to the intersections of buried trenches and, as a consequence, the block corner sections are depressed in comparison with the central section of the block and the level of the emitter apex in the block corner sections is also lowered, which makes emission from the said sections difficult.